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Tip of the Day
Language: Java
Expertise: Beginner
Jul 27, 1998

C/C++ system() equivalent

Question:
Does Java have a command that does what the system() function does in C, or a way to emulate this action? For example, in C, system("ls"); would call "ls" from the command line.

Answer:
Yes, Java supports the execution of shell commands through a method call. The exec() methods of the java.lang.Runtime class are the rough analogs to the C standard library system() function. The behavior of Runtime.exec is a little different though, so you have to be aware of a few things.

First, you have to obtain a reference to the default Runtime instance for your application. You do this by calling the static Runtime.getRunTime() method. Second, you have to call one of the exec() methods with appropriate arguments. This will return a java.lang.Process reference to the executed command.

Where the C system() function blocks until the command finishes, the Runtime.exec() methods do not block. To obtain the blocking behavior, you must call the Process.waitFor(), which will wait for the process to finish executing. In certain respects, Runtime.exec() is more like UNIX's popen(), because after you get the Process reference, you can interact with the input, output, and error streams while the process executes concurrently.

Your example would work as follows in Java (exception handling is omitted):

Runtime runtime = Runtime.getRuntime();
Process process = Runtime.exec("/bin/ls");
process.waitFor();

Keep in mind that whenever you rely on exec(), your code will almost surely not be portable to other systems.

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